There is a technology for disposing an electromagnetic interference suppressor along both of a part, a circuit, a conductive wire, and the like that generate noise (hereinafter collectively referred to as “interfering objects”) and a part, a circuit, a conductive wire, and the like that may be affected by noise (hereinafter collectively referred to as “interfered objects”) in order to suppress electromagnetic interference, in particular, internal interference (so-called autotoxication), occurring in various electronic devices. Electromagnetic interference against the interfered objects can be suppressed through disposition of an electromagnetic interference suppressor in this manner. “Electronic devices” used herein means devices including an electronic part, a circuit, wiring, and the like in general, and examples of the “electronic devices” include wireless communication terminals such as cellular phone terminals, and various computers such as desktop computers, laptop computers, and workstations.
Generally speaking, an electromagnetic interference suppressor is a sheet-shaped part in which magnetic powder is dispersed in a binding agent. The electromagnetic interference suppressor has a high electric resistance, and a frequency-selective loss characteristic resulting from magnetic resonance. Through disposition of the electromagnetic interference suppressor inside electronic equipment, the adverse effect of high frequency noise can be suppressed effectively without electromagnetic disturbance or other secondary side effects. At present, electromagnetic interference suppressors are rapidly becoming popular as an easy noise countermeasure part that “takes effect just by sticking.” Electromagnetic interference suppressors are internationally standardized by IEC 62333.
In Patent Document 1, there is disclosed a technology for enhancing the effect of suppressing electromagnetic interference through disposition of an electromagnetic interference suppressor so that the electromagnetic interference suppressor is in close contact with electronic parts mounted on a substrate. The substrate and the electronic parts set on the substrate are covered with an electromagnetic shield film to be shielded against electromagnetic waves. This close contact of the electromagnetic interference suppressor with the electronic parts and others requires laborious steps to accomplish. In addition, the electronic parts disposed on the substrate and covered with the film together with the substrate cannot adapt to a change to the disposition of the electronic parts when the disposition of the electronic parts is to be changed for some reason or another.
Patent Document 2 includes a description about suppressing interference between electronic parts by attaching an electromagnetic radiation noise absorption sheet near the electronic parts. A point to be noted in Patent Document 2 is that the electromagnetic radiation noise absorption sheet is disposed along an interfering object and an interfered object each. Two electromagnetic radiation noise absorption sheets are illustrated in FIG. 1 of Patent Document 2, as well as in FIG. 2 of Patent Document 2.
According to Non Patent Document 1, main noise suppression functions of a noise suppression sheet, that is, an electromagnetic interference suppressor, are (1) suppression of coupling in the near field, (2) suppression of unnecessary radiation in a high frequency line, and (3) attenuation of a noise component propagated along a transmission line. The following four measurement methods are regulated by IEC 62333-2 to evaluate the main noise suppression functions.
(a) Intra-Decoupling Ratio: Rda
Rda is the proportion of attenuation obtained by attaching a noise suppression sheet in parallel to a transmission line, with respect to spatial coupling occurring between two transmission lines, or between two parts mounted to the same printed wiring board.
(b) Inter-Decoupling Ratio: Rde
Rde is the proportion of attenuation obtained by attaching the sheet in a gap between two transmission lines, between printed wiring boards, or between two parts, with respect to spatial coupling occurring between the transmission lines, between the printed wiring boards, or between the parts.
(c) Transmission Attenuation Power Ratio: Rtp
Rtp is the amount of attenuation obtained per unit line length by attaching the sheet in a transmission line, with respect to a conducted signal or noise propagated along the transmission line.
(d) Radiation Suppression Ratio: Rrs
Rrs is the amount of suppression obtained by attaching the sheet, with respect to radiation noise radiated from a circuit board. This measurement is far-field measurement by the same 10-m method or 3-m method that is used in normal EMI measurement.